Patent GB-1494492-A (GB)
Title: Reading recorded data by means of a monitoring beam
IPC: G06K7/10
Filed: 1974-02-21
Abstract: 1494492 Reading optical record G S B STREET 20 Feb 1975 [21 Feb 1974] 7836/74 Heading G1A A record 14 having lines on a background is read by placing a sheet 15 of photo-sensitive material against the record, directing at the record a monitoring beam which is transmitted or reflected by the lines of the record more than by the background and which does not affect the photo-sensitive material of the sheet 15, and after each line or part line of the record has been read tracing it with an activating beam which activates the photo-sensitive material to change the colour or density of the line or part line when the record 14 and the sheet 15 are viewed in combination, whereby the lines or part lines which have been read can be differentiated from the remainder of the lines. The monitoring beam is provided by a laser 10 and passes to the record via main and auxiliary deflection devices 13, 11 and a lens 12, and the activating beam is provided by a laser 17 and passes to the sheet 15 via a mechanical shutter 18, a modulator 19, a mirror 20 and the devices 11, 12, 13. The record 14 and sheet 15 are examined by directing a projection beam through them and a lens 21 on to a viewing screen (not shown). The main deflecting device 13 comprises a pair of steering mirrors for X and Y co-ordinate deflection, and the auxiliary deflection device 11 may have smaller steering mirrors or electro-optical devices for X and Y co-ordinate deflection, the device 11 providing high speed correction over a small range to the position established by the device 13 which has greater inertia and therefore reacts more slowly. Both devices 11, 13 are controlled by a deflection control circuit device 22, see also Fig. 4 (not shown), under the command of a computer 23 under operator or automatic control. A line scan is superimposed on the movement of the monitoring beam by a raster scan circuit device 24 via the deflection device 11. The monitoring beam is transmitted by the lines  of the record to generate pulses by a photosensor 16 which are applied with the output from the raster scan circuit device 24 to a line position counter 25 which feeds the computer 23 to build up a digitized version of the information on the record 14. During monitoring the shutter 18 is kept closed but during activation of the sheet 15 it is opened and the information in the computer 23 is used via the deflection control circuit 22 and the beam deflection devices 11, 13 to steer the activating beam along the track which has been digitized. The modulator 19 is controlled in dependence upon the velocity of the activating beam across the sheet 15 by an intensity control circuit device 26, see also Fig. 3 (not shown), supplied with a velocity signal from the deflection control circuit device 22 and a density signal from the computer 23 determined by the overall density of the record 14. The activating beam is controlled to provide closely spaced bright dots to provide a line of uniform intensity along the track. The modulator 19 may be a Pockel cell in combination with a Wollaston prism which splits the beam from the laser 17 into two components with different polarizations and which follow slightly different paths. One component is used as the activating beam and its intensity is modulated by rotating the plane of polarization of the incident beam by means of the Pockel cell and thereby varying the distribution of energy between the two components. Alternatively, the modulator 19 may be an acousto-optic modulator using ultrasonic vibrations to control the refraction of the beam by a block through which the beam is transmitted and thereby changes the proportion of the beam transmitted by a stop. The activating beam may be caused to execute a small circular scan and by introducing into its path a diaphragm of smaller diameter than the scan a form of quick-acting electronic shutter is provided for short-time interruption of the activating beam, the control being via the deflection control device 22 and the deflection device 11. The sheet 15 may be a photochromic film placed in front of the record 14 which may be on paper or other non-transparent medium and is scanned by reflected light. If the lines on the record 14 are transparent the sheet 15 may be placed behind the record. Alternatively, the photo-sensitive material may be liquid crystal material which can be changed in colour or intensity by the activating beam. The record 14 may be a diazo film which develops clear transparent lines on a dark background, and the photo-sensitive material may consist of a homogeneous solution of a mercury complex in a polymer coated on to a transparent polymer sheet. The monitoring beam may be provided by a mixed Argon-Krypton gas laser or a Helium-Neon laser, and the activating beam may be derived from the Argon-Krypton laser but of a different wavelength to the monitoring beam and which activates the photo-sensitive material to a dark colour, the lines traced by the activating beam then appearing as darker lines on the dark background of the projected image. The monitoring beam may be controlled to follow the track and its position recorded at intervals. The record may be moved instead of the monitoring and activating beams.